FIX. RadAutoCompleteBox - a NullReferenceException is thrown when press two times space key and enter key in empty editor

To reproduce:
1. Add RadAutoCompleteBox with few items
2. Set the AutoCompleteMode property to SuggestAppend and the WordWrap property to true
3. Start the project and enter 2 times space key. After that press enter key and you will see an exception

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